Electrical prospecting method



Nov. 3, 1942. $M|TH 2,300,709

ELECTRICAL PROSPEKJTIN METHOD Filed Jan. 10, 1941 ,427'0 ays.

Pa tented Nov. 3, 1942 'UNITED STATES. PATENT; OFFICE ELECTRICAL PROSPECTING METHOD George A. Smith, Philadelphia, Pa, assignor to Sperry-Sun Well Surveying Company, Philadelphia, Pa, a corporation of Delaware Applicationjanuary 10, 1941, Serial No. 373,891

2 Claims.

This invention relates to the exploration of the tions; there not only occurs an RI drop due to the resistivity of the liquid in the capillary, but also there appears a counter-electromotive force of such amount that the power required for maintaining the liquid flow is equal to its counter-v electromotive force multiplied by the electric current.

The method as carried out in said Martienssen patent fails to take into account the varying resistivities of the strata encountered by the electrode in the measurement of the counter-electromotive-iorce due to the induced osmosis in the strata of the type described above.

It is the object of the present invention to provide an improved method whereby the resistivity variations are eliminated so that a better picture of the variations in the osmotic potential is secured.

The object of the invention will become clearer from consideration of the followingdescription read in conjunction with the accompanying'drawing, in which the figure illustrates diagrammatically the arrangement of apparatus for carrying out the method.

In the figure, there is illustrated; at 2, a :bore hole penetrating the strata to be explored. As illustrated in the diagram, an electrode 4, making electrical contact with the mud in the borehole,

the point'IZ above ground is not substantially varied as varying currents are drawn through the electrode circuit. As a matter of fact, the currents used in accordance with the present invention arerelatively small, ofthe order, for example, of a milliampere or less, and the meter i4 is therefore of a type adapted to indicate currents of that order.

' The resistance It is used to eliminate resistivity variations from consideration, andfunctions as will be clear from the following contiderations.

Designating by Re the total external resistances in the circuit, i. e., the total resistances of the resistance I 6, the meter l4 and the cable (the is supported by means of an insulated cable 5 for movement through the bore hole. At the surface there is provided a constant source of electromotive force 6, for example, a storage battery, across which is connected a potentiometer resistance 8 grounded at one'end as indicated at l8. The adjustable slider i2 contacting this resistance is connected to the cable 5 through a meter [4 and a relatively high resistance IS.

The arrangement is such as to provide a substantially constant potential between I! andthe ground, and to this end the potentiometer relatter two of which will be normally negligible relative tothe resistanpe l6), and the resistance appearing at the electrode R0, and designating the counter-electromotive force produced'by induced osmosis as E, and naturally occurring potentials due, for example, to electrochemical action or electroflltrationconditions, as Ex, then the current I flowing through the electrode will be given by the equation- The above equation indicates the dependence of'the current upon the resistance at the electrode, i. e., it will vary in accordance with that resistance, and if Re was negligible, as in the case of the method described in said Martienssen patent, the current would be substantially inversely proportional to changes in the resistance at the electrode as the electrode moved through the hole. I

Differentiating the above equation:

d(E.+E.) [E-(Et+E. 1dR.

R,+R., (RtXRt) If Re Ro, the above becomes to a close approximation:

8 It will thus be seen that if at is made sumciently large, the effects of B0 on the current-P may be eliminated'asiar as desired. In accord- -ance with the present invention, thereiore,.Re

' is made relatively high compared to the highest resistance appearing atthe electrode during its passage through the strata... For. a given electrode and locality,. this maximum resistance which could be expected to be encountered will be known. It, therefore; Re is made, say, one

sistance 8 must be so low that the potential of 55 hundred times such maximum resistance, the

the electrode.

changes in current due to the maximum variations of resistance at the electrode may be made less than one per cent. 01' the variations noted.

If a higher accuracy independent oi the resistance to variations is desired, it is only necessary to make the external resistance Re correspondingly larger.

- segregate such variations from the eflect's which It will be noted that such increase of Re does Q not render the current changes any less proportional to the changes of the potentials appear-' ing at the electrode. Of course, as Be is made greater, a more sensitive meter is required, but meters having full scale sensitivity of 100 microamperes or less are quite rug ed and portable, so that increase in Re is not in the least disadvantageous.

The potential applied to the circuit between points Ill and I2 is not particularly important, except from the standpoint that the constant potential there applied should not be so large as might be due to resistivity changes, which are or very substantial magnitude and would serve to mask completely the electro-osmotic poten-v tials.

The meter ll may be of recording type, ar-

ranged, for example, to record its indications shows means for recording meter readings apto unduly increase the value of the second term of the differential equation giv n above, which. as will be evident, will be negligible if Re is much greater than Re, unless the value of E is abnormally increased. v As a guide, it may be said that E should have a value of the same order of magnitude as the electro-osmotic 'fiow potentials due to the porosity of the adjacent strata, although it maybe increased substantially above this if desired. The disadvantage of increasing it too greatly is that the meter l4 must be chosen of a capacity dependent upon this potential, and it it is too large, the observed variations will occupy only a small portion of its scale. If the meter is not simply a current measuring meter but includes a suitable bucking circuit, a larger magnitude of the supply potential is not disadvantageous.

plicable to a metersuch as used herein and correlating the indications with depth. The method of this invention may, 01' course, be combined with other logging methods including those determining high frequency characteristics of strata, resistivities, acoustic characteristics, etc. The electrode I is desirably of conventional non-polarizing type and may be the cathode or anode of the system.

What I claim and desire to protect byLetters Patent is:

1. The method of determining porosity char acteristics of strata traversed by a bore hole comprising moving through the bore hole an'elec- As indicated in the final equation given above,

the variations observed are not solely due to the variations in the electro-osmotic potential,-but also involve, in algebraic summation, variations in other natural potentials which may appear at In general, however, these other potentials arev of a lower orderoi magnitude than in the electro-osmotic potentials, and the variations incurrent observed are, in a relatively minor degree, dependentfthereon, but for the practical purpose of electrical logging, this is quite immaterial, since electrical logs are generally used for comparison purposes rather than for the purpose of making absolute measurements, and they are always interpreted by those knowledge gained from experience. It is not, therefore, the object-of the present invention to skilled in the art on the basis of empirical segregate completely the changes due to electroosmoticpotentials from changes due to other locally occurring potentials, but primarily to trode, causing a flow of current through strata surrounding the electrode by applying to the electrode, through a resistance .of a substantially higher order than any resistances appearing at the electrode during its passage through the strata, a substantially constant potential of the same order-oi magnitudeas the electro-o'smotic flow potentials due to the porosity of the adjacent strata, and observing the changes of intensity of the electrode current as a measure of the porosity characteristics while the electrode traverses the strata. I

2. The method of determining porosity characteristics or. strata traversed by a bore hole comprising moving through the bore hole an electrode, causing a flow of current through strata surrounding the electrode by applying to the electrode, through a resistance of substantially higher order than any resistances appearing at the electrode during its passage through the strata, asubstantially constant direct potential, and observing the changes of intensityof the electrode current as a measure of the porosity characteristics while the electrode traverses the strata. GEORGE A. SMITH.

williston applica- 

